lister/tasks: Standardize return statements

The following commit adapts the return statements from both lister and their
associated tasks. This standardizes on what other modules (e.g. both dvcs and
package loaders) do.

swh/lister/cgit/lister.py

@@ -78,6 +78,7 @@ class CGitLister(ListerBase):
         }
 
     def run(self):
+        status = 'uneventful'
         total = 0
         for repos in grouper(self.get_repos(), 10):
             models = list(filter(None, (self.build_model(repo)
@@ -87,6 +88,9 @@ class CGitLister(ListerBase):
             self.db_session.commit()
             total += len(injected_repos)
             logger.debug('Scheduled %s tasks for %s', total, self.url)
+            status = 'eventful'
+
+        return {'status': status}
 
     def get_repos(self):
         """Generate git 'project' URLs found on the current CGit server

swh/lister/cgit/tasks.py

@@ -10,7 +10,7 @@ from .lister import CGitLister
 @shared_task(name=__name__ + '.CGitListerTask')
 def list_cgit(**lister_args):
     '''Lister task for CGit instances'''
-    CGitLister(**lister_args).run()
+    return CGitLister(**lister_args).run()
 
 
 @shared_task(name=__name__ + '.ping')